U.S. Pat. No. 5,008,432 describes the addition of H.sub.2 S to unsaturated compounds, such as methyl acrylate or acrylic acid.
This addition is carried out in the presence of a basic catalyst chosen from magnesium oxide and basic anion-exchange resins. These resins are chosen from those having tertiary amines or quaternary ammonium hydroxides as functional group.
The reaction takes place in the absence or in the presence of solvents. The latter are chosen from lower alcohols, saturated aliphatic or cycloaliphatic hydrocarbons and aromatic hydrocarbons.
If an anion-exchange resin is used, the reaction pressure is generally from 3037.5 to 6750 kPa.
Example VII describes the addition of H.sub.2 S to acrylic acid, without solvent, at a reaction pressure of 3037.5 kPa, in the presence of Amberlyst A-21 resin (Rohm and Haas). This resin has dimethylamino functional groups. During the reaction, a solid separate from the liquid medium is formed. This solid contains mercaptans, sulphides, acrylic acid and dimers and trimers. Test No. 4 of this example would result, for an H.sub.2 S/AA molar ratio of 10.4, in a conversion of 89% and a selectivity of 100% for MPA.
Test No. 3 would result, for an H.sub.2 S/AA molar ratio of 5.4, in a conversion of 90% for a selectivity of 98% for MPA.
Patent application J07-228568 also relates to the synthesis of MPA by addition of H.sub.2 S to acrylic acid. According to the process of this application, the addition is carried out in the presence of an anion-exchange resin and of a solvent chosen from water or amide, ester, ether or ketone compounds.
The amide, ester, ether or ketone solvent must not have a hydrogen bonded to an oxygen, sulphur, nitrogen and similar atom. Among the solvents which may be used in this process, dimethylformamide (DMF), dimethylacetamide, N-methylpyrrolidone and dimethylimidazolidinone are recorded among the amide solvents, DMF being preferred as it results in a high MPA yield.
Dioxane, dioxolane and diethylene glycol dimethyl ether are recorded among the ether solvents, dioxane being preferred as it results in a high MPA yield.
Acetone, diethyl ketone, methyl ethyl ketone and methyl isobutyl ketone are recorded among the ketones.
The anion-exchange resin described can have, as functional group, a tertiary amine (weakly basic resins) or a quaternary ammonium hydroxide (strongly basic resins). The weakly basic anion-exchange resins are indicated as ideal in practice because they do not form salts with the compounds of the reaction mixture.
The polymer of these resins, rendered insoluble by crosslinking, can be polystyrene, polyacrylamide or an epoxy resin.
In J07-228568, the examples of the synthesis of MPA employ the following resins manufactured by the company Rohm & Haas:
Amberlite IRA 93 (groups: tertiary amine) PA1 Amberlite IRA 94 (groups: tertiary amine) PA1 Amberlite IRA 900 (groups: quaternary ammonium) PA1 1) a guanidine radical of general formula (C): ##STR1## in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are, independently of one another, hydrocarbon groups such as methyl, ethyl, propyl or butyl, the imine nitrogen being bonded to the solid support via a chemical bond or a sequence of chemical bonds, PA1 2) a bicyclic guanidine radical of formula (D): ##STR2## in which n has the value 2 or 3 and m has the value 2, 3 or 4, provided that n is less than or equal to m, this radical (D) being bonded to the solid support via a chemical bond or a sequence of chemical bonds starting from the initially N--H nitrogen of the corresponding bicyclic guanidine. PA1 the radical (C) is substituted by L, the latter then representing a --CH.sub.2 -- radical and R.sub.1, R.sub.2, R.sub.3 and R.sub.4 each representing a methyl group, PA1 the radical (D) is substituted by L on the nitrogen which, in the related bicyclic compound, carries a hydrogen, provided that L then represents a --(CH.sub.2).sub.p -- radical, p being an integer having a value from 1 to 9. PA1 1) The group B is a radical of general formula (C). PA1 a) The starting point is the preparation of a PS-DVB resin functionalized by primary amine groups having the general formula (A) ##STR7## PA1 1) It is possible, for example, to start from a resin of general formula (J): ##STR8## X being a leaving group, in particular halogen or tosylate, obtained from a hydroxyl group --OH, and L representing in particular a --(CH.sub.2).sub.p -- radical with p being an integer having a value from 1 to 9. PA1 2) It is also possible to start from a PS-DVB resin of formula (J) in which L represents a methylene and X has the above meaning and preferably represents a chlorine atom. Applicant has found that this chloromethylated resin can be reacted with an alkanolamine or a mercaptoalkylamine, in the form of an alkali metal alkoxide or thiolate, under the conditions of the Williamson reaction. PA1 The alkanolamine or the mercaptoalkylamine, diluted in anhydrous tetrahydrofuran (THF) or anhydrous N-methylpyrrolidone, is reacted with sodium hydride in suspension in the same anhydrous solvent. After formation of the sodium alkoxide or sodium thiolate, the chloromethylated resin is introduced into the liquid reaction mixture. PA1 b) After obtaining the resin possessing primary amine groups of general formula (A), these primary amine groups are reacted with chloroformamidinium chloride (Vilsmeier salt) of general formula (H): ##STR9## in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are, independently of one another, chosen from the methyl, ethyl, propyl and butyl groups, PA1 in order to obtain a PS-DVB resin functionalized by a guanidine group and with the general formula (I.C): ##STR10## L and R.sub.1 to R.sub.4 having the same meanings as above. PA1 2) In the general formula (I), the group B is a radical of formula (D)
Example 2 (Amberlite IRA 94 resin) and Example 12 (Amberlite IRA 900 resin), carried out under identical conditions as regards the molar ratio (H.sub.2 S/AA=3) and the reaction temperature of 60.degree. C., would result virtually identical MPA yields (90.0% and 89.7%) and selectivities (90.6% and 90.8%).
Example 3, carried out in DMF in the presence of Amberlite IRA-94 resin, would result in a conversion of the AA of 98.9% and an MPA yield of 91.5% with a selectivity of 92.5%, for an H.sub.2 S/AA molar ratio of 6.0, a reaction temperature of 60.degree. C., a pressure of 30 atm (3039 kPa) and a maximum pressure of 44 atm (4458 kPa).